[28.07] Forming Well-compacted Meteorites in the Solar Nebula

Chondrites are well-compacted stones with porosities
generally less than 10%, mostly as post-lithification
cracks. Pressure, temperature, and water lithify terrestrial
rocks; but though all meteorites have experienced some
metamorphism, most have not seen sufficient pressure,
temperature or aqueous alteration to account for their
lithfication. How did nebular dust become well-lithified
meteorites?

Pressures of 1-10 GPa are needed to compress terrestrial
sandstones; 1 GPa pressures are reached only at the center
of a Ceres-sized body. However, relative impact velocities
of 1 km/s, equivalent to orbits with eccentricity of circa
0.05 in the asteroid region, have an energy density
equivalent to a GPa.

When particle velocities are controlled by nebular gas drag,
the maximum impact velocity equals the deviation of the gas
from Keplerian rotation. Millimeter-sized particles coupled
to the gas impact on meters-sized or larger bodies at
roughly 50 m/s, compressing underdense "fairy castle"
structures, though hardly lithifying them. Such bodies would
be coherent enough to participate in further accretion,
however.

Jupiter forming in the nebula (perhaps also inducing shocks
to form chondrules) can perturb a 100-km planetesimal, even
if not near a major resonance, to eccentricities fluctuating
from 0 to 0.1 (resonant bodies attain much higher e's).
Ten-km bodies should attain eccentricities of 0.05, while
smaller ones would be damped to low eccentricity until the
gas dissipated.

Collisions of such perturbed bodies at speeds many times the
target body's escape velocity would disrupt similar-sized
bodies, while collisions with smaller impactors would allow
the target to survive. A series of such impacts could
produce lithified regions in a more porous unconsolidated
matrix. Subsequent collisional disruptions would dissipate
this matrix, but allow the lithified regions to survive to
the present.

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